We should get Mike Brown hooked on Gravity Simulator. My effort to reproduce his graphic yields a slightly more accurate one. In his graphic Uranus' orbit should be more elliptical, and Ceres shouldn't produce such a fat orbit trail. His table of objects includes Sedna, but his diagram doesn't show it.

Here's my effort using Gravity Simulator and the list of objects in his table:

Final Version of Resolution on the Definition of a Planet At the second session of the General Assembly which will be held 14:00 Thursday August 24 in the Congress Hall, members of the IAU will vote on the resolutions presented here. There will be separate sequential votes on Resolution 5A and Resolution 5B. Similarly, there will be separate votes on Resolutions 6A and 6B. Resolution 5A is the principal definition for the IAU usage of “planet” and related terms. Resolution 5B adds the word “classical” to the collective name of the eight planets Mercury through Neptune. Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the proto-type. Resolution 6B introduces the name “plutonian objects” for this class. The Merriam-Webster dictionary defines “plutonian” as: Main Entry: plu•to•ni•an – Pronunciation: plü-’tO-nE-&n – Function: adjective – Usage: often capitalized – : of, relating to, or characteristic of Pluto or the lower world.

Resolutions Committee members will be available at the IAU Exhibit (situated in the exhibition area, 2nd floor of Congress Hall, Foyer 2) from 13:00–13:30 today (Thursday). However, only minor corrections can be accommodated at this stage. A French version of the Resolutions will be available at the door. IAU Resolution: Definition of a Planet in the Solar System Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation ‘planets’. The word ‘planet’ originally described ‘wanderers’ that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

Resolution 5A

The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet[1] is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape[2],

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite.

(3) All other objects[3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. [2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories. [3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Resolution 6A The IAU further resolves: Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

Resolution 6B The following sentence is added to Resolution 6A: This category is to be called “plutonian objects.”

250 words for Compromise. Achieving a planet definition has been all about compromise. There are two equally valid descriptions of what should be the principal criterion for defi ning a planet. One is dynamical, an object that has “cleared out its zone.” The other is based on the physical nature of the body itself. The pendulum of argument has swung both ways during the General Assembly discussions. But now it has swung too far. Resolution 5B is all about finding the middle ground. Using qualifiers gives equal status to both points of view and leaves open the possibility to define other types of planets in our Universe. Resolution 5B restores the “global and cultural points of view” that the Planet Definition Committee had responsibility to achieve. The public recognizes Mars, for example, as a “planet” not because it has cleared out its zone, but because it is a fascinating world. To illustrate why Resolution 5B is cultural, and not silly semantics, consider how you must answer two questions: How many planets are there? Is Pluto a planet? A vote in favor of 5B yields: “There are 8 classical planets and many dwarf planets yet to be discovered” and “Pluto is a planet, but in the dwarf planet category.” These answers highlight and communicate the tremendous revolution of new discoveries in our outer Solar System. Further, it saves enormous public backlash by still being able to say the words “Pluto is a planet, but”. Do not underestimate the global cultural importance of these fi rst four words. The word “planet” deserves to be shared equally.

250 words against Resolution 5B represents a small but significant change to Resolution 5A. The key issue is the definition of “planet”. Resolution 5A is close to the version agreed by consensus on Tuesday evening where it was made clear that three distinct categories of objects orbiting the Sun were being defined: planets, dwarfplanets, and small bodies. The logical implications from the rules of grammar cannot be ignored. By using the name “planet” with two different adjectives “classical” and “dwarf” a larger category of planets is implied. This contradicts the first paragraph of both Resolutions 5A and 5B and transforms three distinct categories into two (planets and small bodies) and two sub-groups of planets. To the question “is Pluto a planet?” the two resolutions give different solutions – “Yes” for 5B and “No” for 5A. To the question “How many planets are there?” Resolution 5A gives 8, Resolution 5B currently gives 12 and soon at least 50. The total number of planets may not matter to scientists, it is critical for education and the dissemination of science. For scientists, it is relevant that dynamical and cosmogonical criteria, which are now the source for the defi nition of planets, would in Resolution 5B be relegated to a secondary role. In Resolution 5A the arguments from geophysics and from dynamical astronomy are given equal weight. Such a balanced solution had received very strong support in the meeting of Division III (Planetary Systems Science) and the Planet Definition Information Meeting. Resolution 5B is misleading and should be rejected.

I notice that in the definition voted on here, a planet is greater than 50% of the rest of the material in the orbit. The paper says >10000 makes sense, however there would be pathological cases, such as Neptune = 8500> Pluto, which is a result of there being nothing (known) outside of Neptune to perturb plutinos back into Neptune, such as Neptune did for Uranus. They suggest >100 x the mass of the rest of the stuff in similar orbits, including (I presume) 1:1, 2:3, 3:5 etc. resonances. That's reasonable.

For our solar system.

We're in trouble for those stars that have suffered some kind of major chaotic disaster, and of the ones discovered so far, this seems to be a majoritiy of the cases! I thought this definition would be general ??? .

...which is a result of there being nothing (known) outside of Neptune to perturb plutinos back into Neptune, such as Neptune did for Uranus...

That's a very interesting point. I don't believe Uranus has any known resonant objects. But after a quick search through the paper for "Uranus" I didn't see this mentioned in the paper. Was that your conclusion?

Yes, indeed it was my conclusion that Neptune perturbed Uranus' resonant objects back into Uranus , or into other planets (including Neptune) in a few cases. I'll own up to that. Sorry if I left an impression that the paper said that.

I have done a whole bunch of GravSims of Neptune resonances. So I've got a pretty good feel for objects that get thrown out of Neptune's orbit. If there was another planet beyond Neptune, the ones thrown to the outside would not be stable. Since there's nothing substantial outside of Neptune then Neptune became a pathological case. But it still outweighs everything in it's sphere.